Add "experimental and untested" warnings to seismic.wavefd

[leouieda]

Fixes #310

Added a warning message to the module docstring and a warnings.warn call that is triggered at import time. This way people know that this code is not for research purposes.

Checklist:

• Make tests for new code (at least 80% coverage)
• Create/update docstrings
• Include relevant equations and citations in docstrings
• Docstrings follow the style conventions
• Code follows PEP8 style conventions
• Code and docs have been spellchecked
• Include new dependencies in doc/install.rst, requirements.txt, environment.yml, ci/requirements-conda.txt and ci/requirements-pip.txt.
• Documentation builds properly (run cd doc; make locally)
• Changelog entry (leave for last to avoid conflicts)